During imaging (for example, imaging of laser-sensitive media), the interaction of the laser and the media causes a physical and/or chemical change to the imaged areas of the media. In this process, matter may be expelled from the media. In some cases, this matter comprises the by-products of intentional imagewise ablation of the media. In other cases, this matter comprises unintentionally expelled imaging by-products. The by-products expelled during imaging may generally include particulate debris and/or gaseous emissions.
The expelled matter is typically collected and drawn away from the vicinity of the imaging laser by a nozzle of some type which is connected to an airflow created by an air mover (e.g. a blower). The airflow containing the expelled matter is subsequently passed through a filtration system to remove particulate and gaseous emissions from the airflow. An example of such a system is the Debris Removal System sold by Creo Inc. of Burnaby, British Columbia, Canada, as an accompaniment to various imaging devices also produced by Creo Inc., such as the Trendsetter™ and Trendsetter VLF™ imaging systems. Depending on the nature of the expelled by-products, the filtration may include one or more stages of particulate filtration and/or chemical filtration. Chemical filtration is usually accomplished by adsorption of gaseous by-products by solid sorbents such as activated carbon, for example.
U.S. Pat. No. 4,751,501 (Gut) describes a system for detecting a clogged particulate filter. The system includes a detection circuit for detecting a pressure drop across a filter and a sensor for sensing airflow through a duct. A comparison circuit is connected to the pressure drop detector and the airflow sensor to provide an output which indicates whether the filter is clogged.
Monitoring the end of the service life of chemical filters has been previously accomplished by specifying in written instructions a time of service after which the filter should be replaced or a weight increment. In some cases, users detect the end of the service life of chemical filters by smelling materials which have passed through the filter and which should have been trapped in the filter.
Most known methods for detecting the end of service life of chemical filters do not directly assess remaining sorbent capacity. Those known methods, which do attempt to assess the sorbent capacity, are typically impractical for use in the by-product removal systems of media imaging devices. In general, it is desirable to use filters to the maximum of their capacity and at the same time to ensure that their service life is not exceeded. Furthermore, it is desirable to have some indication of the integrity and proper functioning of the filtration system as a whole. The ability to assess the remaining sorbent capacity of a filter may help to achieve these objectives.
A particulate filter typically becomes filled with debris as it filters particles from an airflow. The end of the filter's service life may be determined by measuring a pressure drop across the filter. For example, the filter may be deemed to have reached the end of its service life when the pressure drop increases beyond a predetermined pressure drop limit. Such a pressure drop monitoring technique may be sufficient in a filtration system where the airflow is constant, but such techniques are not effective where the flow rate is variable, as is often the case in systems for collecting particulate by-products produced by laser imaging systems.
Determining that a particulate filter is at the end of its service life when a pressure drop across the filter reaches a predetermined pressure drop limit, regardless of the airflow, may substantially underestimate the capacity of the filter. This technique does not account for variation in the air mover performance, which may occur because of voltage or frequency variation. For example, when the air mover is driven by a DC brushless motor, voltage variation can affect its performance and when the air mover is driven by an induction motor, frequency variation can affect its performance. Similarly, the technique of determining filter service life using the pressure drop over the filter regardless of air flow does not account for variation in the resistance to air flow which may arise, for example, if a hose is crushed, kinked, or bent. With the same amount of particulate debris in the filter, a higher airflow causes a greater pressure drop across the filter, possibly resulting in a premature indication that the predetermined pressure drop limit has been reached. Because of these factors, a substantial error in determining filter service life may occur.